1. Field of the Invention
The present invention relates to a negative voltage converter, and more particularly to a negative voltage converter using P-type thin film transistors (P-type TFTs).
2. Description of the Related Art
Referring to FIG. 1 and FIG. 2, FIG. 1 is a circuit diagram of a negative voltage converter 10 using P-type TFTs according to prior art. FIG. 2 shows a relationship of voltage applied on a node X depicted in FIG. 1 varied with a threshold voltage Vth of the transistor 16. The Negative voltage converter 10, using P-type Thin Film Transistors (P-type TFTs), is capable of transforming an input positive voltage or zero voltage into a negative direct current (DC) voltage to be desired by a circuit or a component. The negative converter 10 comprises an input circuit 12 and a voltage amplifying circuit 14. The input circuit 12 comprises a P-type TFT 16 and a capacitor 18 electrically coupled to a clock signal CLK. The voltage amplifying circuit 14 may be a P-type TFT. Input of the input circuit 12 is 0V, and clock signal CLK is a square wave with amplitude between 0 and 5V.
However, a difference in P-type TFT fabrication process may result in different threshold voltage of P-type TFT. As can be seen in FIG. 2, the curves 51, 52, 53, 54 respectively represent waveforms on the node X corresponding to various threshold voltages −1V, −2V, −3V, −4V. Obviously, different threshold voltage Vth causes a significant output deviation in output waveform on node X.
Referring to FIG. 1 in conjunction with FIG. 3, FIG. 3 illustrates the negative voltage converter 10 producing output voltage VOUT based on various threshold voltages. As shown in FIG. 3, the curves 61, 62, 63, 64 respectively represent waveform of voltage VOUT dependent on various threshold voltage of P-type TFT of −1V, −2V, −3V, −4V. If the voltage applied on the node X is a square wave with 0˜−5V, the difference of the output voltage VOUT varied with different threshold voltage is approximately 3V. In other words, output of each input circuit 12 and voltage amplifying circuit 14 is varied as the threshold voltage of the P-type TFTs fabricated by using different processes. As a result, a deviation of output voltage is accumulated as a use number of input circuits 12 and voltage input circuits 14.
For example, two negative voltage converters are used in a circuit, and each converter comprises an input circuit and two voltage amplifying circuits. One converter employs P-type TFTs has a threshold voltage of −1V and the other employs P-type TFTs has a threshold voltage of −4V on account of different fabrication processes. Since an output deviation of each input circuit and voltage amplifying circuit is approximately 3V, a total deviation of output voltage of the entire negative voltage converter 10 will increase up to 9V. Such deviation directly influences on an operation subsequent stage of circuits and components, increasing complexity of design, and reducing yield of products accordingly.
Therefore, it is necessary to develop a negative voltage converter capable of supplying a negative voltage independent of a threshold voltage of P-type TFTs.